ample space on an airplane, buy a first-class ticket.

I know it's easy to tell the large person to spend more money to buy two coach seats (rather than asking you to spend more money to get a first-class ticket), but the reality is that both of you want more space than you are currently getting, so it's not clear to me which one should be asked to spend the money to get the space they desire.

I understand that the large person is the one who "caused" the issue to begin with. But I don't think there is any other form of public transportation where we expect large people to spend more money to ensure our own comfort. Why is it different on an airplane?

In general, I don't think I'm compelled by the underlying premise many have that when you buy a plane ticket, you are entitled to a certain amount of space on the plane, as if each passenger purchased 3 cubic feet of space and is bickering with anyone who crosses their invisible line of personal space (the way siblings bicker with each other when their brother invades their personal space). It's public transportation. You bought the opportunity to travel from location X to location Y. That's it. Of course the airline has to put your butt in a seat because that's the law. But that doesn't automatically grant you rights to all the cubic airspace surrounding your seat, just as your seat on a subway doesn't guarantee you anything, other than the right to travel from location X to location Y.

If you want to decrease the odds of another passenger making you uncomfortable, purchase a ticket for the first-class cabin where there will be fewer passengers and you will likely have ample space.

As an aside, one of the underlying issues here is the general disdain many have for fat people. Generally speaking, there is not nearly as much sympathy for fat people as their is for people dealing with other issues, even though obesity is often a manifestation of some other issue (rather than a sign of laziness or a lack of discipline), and even though many of the issues healthy people struggle with are also self-imposed and inconvenient to those around them.

Note: No, I do not take up much room on an airplane.